JANUARY 2017
Some elements of the title illustration are designed by Freepik
How do plants keep in touch?
Keep
in
touch?
Hello!
Authors:
Dimitrije Markovic, Neda Nikolic,
Robert Glinwood, Gulaim Seisenbaeva,
Velemir Ninkovic
Associate editor:
Seda Dawson
Abstract
If someone behind you tapped you on the shoulder, your
first response would probably be to turn around. All living
organisms respond to stimuli, and plants are no exception
to this rule. Since they cannot turn around, though, how
would they respond to a light contact with neighboring
plants? Are plants in touch with their neighbors? We
searched for the answer to this question.
We applied a light touch to potato plants and measured
changes in their trichomes (plant’s hair), biomass
Introduction
Just like humans, living with others can be hard for plants. Unlike
humans though, plants cannot pack up and move to another
place. To survive, they sense and respond to the cues from their
neighbors. Changes in the amount of sunlight or the chemicals
in the soil give plants cues about their neighbors. Plant leaf tips
may also touch with neighboring plants due to wind or lack of
space. This mechanical stimuli may be another way plants gain
information about their neighbors.
Plants respond to mechanical stimuli in various ways. For example,
a very small group of plants, including Mimosa pudica and Venus
fly traps, respond very rapidly. A Venus fly trap shuts its traps,
whereas the Mimosa pudica folds inward when touched. Most
plants, however, change their structure or physiology over longer
periods of time.
Plants are the basis of all food webs. A change in plants can impact
their interaction with other organisms. For example, herbivorous
distribution, and the volatile compounds they released. We
also analyzed changes in plant structure, physiology, and
interactions with insects.
Our results showed that plants do respond to light touch
by their neighboring plants. This affects their structure in a
way that makes them shorter and stockier compared to the
untouched plants. Curiously, insects do not find the smell of
touched plants very attractive and prefer their untouched
counterparts.
insects are very sensitive to the changes in plants. A change in
a host plant’s status may result in an increase or decrease in the
population of herbivorous insects. This may have consequences
for agricultural crop production.
In this study, we used potato plants to investigate the effects of
touching by neighboring plants. We studied the following changes
in touched vs. untouched (control) plants:
1.
2.
3.
4.
Biomass distribution
Amount of trichome (plant’s hair)
Released volatile chemical compounds
Their effect on odor preference of herbivorous insects (Fig. 1)
Figure 1:
A winged peach-potato aphid
(Myzus persicae). It is hazardous
for plants because it acts as a
vector for the transport of plant
viruses, such as potato virus Y and
potato leafroll virus to members
of the nightshade/potato family
Solanaceae. (Source: Wikipedia)
Мore free environmental science resources аt: www.ScienceJournalForKids.org
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JANUARY 2017
HOW DO PLANTS KEEP IN TOUCH?
Methods
We used an important vegetable crop for human nutrition
- the potato (Solanum tuberosum) - as our model plant
(Fig. 2). The two most harmful pests to the potato are
potato aphid and green peach aphid. We used these two
species of aphids as the herbivorous insects of our study.
To simulate contact with a neighboring plant, we used a
soft brush to touch the ends of the leaves of each potato
branch for 1 min per day (we call these plants “treated”)
(Fig. 3). After the 8th day, we collected air samples
with the volatile compounds released by the plants and
identified the gases.
We used another group of plants to study plant-aphid
interaction. We placed the aphids in a special chamber
where they could smell both touched and untouched
plants. To determine aphids’ smell preferences, we
counted the number of visits by each aphid to the treated
and untreated plants.
We started structural analyses after the 17th day of the
touching treatment. We picked the plants and measured
stem, branch, and leaf mass of each of them. We
calculated the mass ratio of each part (e.g. What fraction
of the entire mass does the stem constitute?)
We collected the touched terminal leaves and primary
leaves of the same branch. We analyzed the structural
changes on leaf surfaces by determining amount of
trichomes.
flower
inflorescence
fruit
leaflets
compound leaf
main stems
lateral stem
mother tuber
tubers
stolons
roots
Figure 2:
A potato plant. (Source: International Potato Center)
Results
Our data showed that touching resulted in a decrease
in plant height and stem mass fraction but an increase
in branch and leaf mass fraction. As a result of these
changes, touched plants were shorter with more compact
appearances compared to the control group.
Regarding the trichomes, we found that treated terminal
leaves did not show a significant change. However, the
Figure 3:
A researcher is touching the treatment group of plants with a fine brush.
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JANUARY 2017
HOW DO PLANTS KEEP IN TOUCH?
primary leaves on the same branches showed a significant
increase in number of trichomes.
We detected 16 different volatile compounds released by
potato plants. The total amount of compounds released
by touched and control plants was equal but the exact
amount of each of the compounds was different.
Both aphid species showed a significantly lower preference
for the smell of touched potato plants compared to
untreated plants (Fig. 4).
Figure 4:
The effects of touching on aphid’s behaviour. The
difference we observed was statistically significant.
Discussion
Plants are clearly in touch with their neighbors. They
reprogram themselves in many ways when they sense their
existence by a light touch, but why?
Let’s analyze structural modifications first. Touched plants
changed their biomass distribution and became more
compact than others. This is a common response to
mechanical stimuli among plant species and makes them
more resistant to potential damage. For example, plants
that live in very windy environments tend to have shorter
and stockier trunks than the plants of the same species that
grow in less windy environments. Furthermore, compact
plants may have a better chance of survival when they need
to compete for space with neighboring plants.
Regarding physiological modifications, an increase in leaf
surface and number of trichomes helps plants use sunlight
and water more efficiently. This may be quite helpful
when resources are scarce and gives plants a competitive
advantage over their rivals.
What about plant-insect interactions? Aphids and other
herbivorous insects use their sense of smell to locate
host plants. In our study, changes in the blend of volatile
compounds altered the smell of touched plants and made
them less attractive to aphids.
Conclusion
Scientists use cause and effect analysis to understand
complex relationships between organisms. As we have seen
in our results, a light touch between plants can induce (cause)
changes in their structure and physiology. Furthermore, by
making plants less attractive, these changes may extend to
organisms at higher trophic levels.
Could plant responses to mechanical stimuli have ecological
and evolutionary significance? Possibly but answering this
would need further research. Now the big question is: Could
a stimulus, as subtle as touch, lead to evolutionary changes
in plants and organisms higher up on the food chain? Did the
evolution of potato plants just unfold in front of our eyes?
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JANUARY 2017
HOW DO PLANTS KEEP IN TOUCH?
Glossary of Key Terms
Control group − A control group in a scientific experiment is a group separated from the rest of the experiment where the
independent variable being tested (in this case - being touched) cannot influence the results. This isolates the independent
variables’ effects on the experiment and can help rule out alternate explanations of the experimental results. In this case, the
control group were the untouched plants.
Biomass − The mass amount of living matter.
Stimulus (plural. stimuli) − Something that causes a change or a reaction.
Heat and light are physical stimuli.
The dog responded to the stimulus of the ringing bell.
Physiology − The way in which a living organism or bodily part functions.
"the physiology of the brain".
Primary leaf − The first pair of leaves that emerge after the germination of seed.
Terminal leaf − Leaf at the top of a stem or branch.
Trichome − An epidermal hair structure on a plant. Some of these structures contain volatile oils and other secretions that are
produced by the plants.
Volatile compounds − Organic compounds that easily become vapors or gases.
Check your understanding
1
Plant response to the mechanical stimuli of touch by neighboring plants is analyzed in this
study. Can you think of other stimuli that causes plants to respond in certain ways?
2
What are the effects of touch on plant structure and physiology? How do these effects help
plants compete with their neighbors?
3
4
Why was it important for the scientist to study the effects on plant-insect relationships?
How can we use this information in regards to growing crops?
REFERENCES
Markovic D, Nikolic N, Glinwood R, Seisenbaeva G, Ninkovic V (2016) Plant Responses to Brief Touching: A
Mechanism for Early Neighbour Detection? PLoS ONE.
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0165742
Braam J. (2005) In touch: plant responses to mechanical stimuli. New Phytologist.
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2004.01263.x/full
Plant responses to stimuli - teacher’s resource.
http://leavingbio.net/plant%20responses.htm
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